The present invention relates to a process for manufacturing a porous ceramic filter using silicon powder as a raw material and methyl cellulose as a binding agent.
In general, a porous ceramic filter can be used in metallic smelting processes to filter off impurities in molten metals. In the past, the material used for making ceramic filters was aluminium oxide, but it is only suitable for filtering molten aluminium alloys due to its low resistance to heat. If Si.sub.3 N.sub.4 (silicon nitride) is used as a raw material, the thermal shock resistance of the ceramic filter produced is three times that of filters using aluminium oxide, therefore, a Si.sub.3 N.sub.4 ceramic filter can be utilized in stainless steel or superalloy processes with operating temperatures higher than 1400.degree. C.
PCT No: WO 88/00933 discloses a method for fabricating a porous ceramic filter with silicon powder combined with silicon dioxide as a starting material. After injection molding or extrusion molding, the organic additives in the material are burnt off at 800.degree. C., then the ceramic composition is nitrided to silicon nitride. However, the process is complicated since several kinds of organic compounds are added and high temperatures (e.g.800.degree. C.) are required to totally burn off the additives.
German Patent Publication No. DE 38 35 807 uses silicon nitride as starting material, and adds aluminium oxide, yttrium oxide, silicon oxide, titanium dioxide, magnesium oxide, zirconium oxide and chromium oxide as sintering agents. After a ceramic slurry is readily prepared, it is adsorbed to a foamed polymer to form green ware. Finally, the green ware is dried and sintered at 1500.degree.-1800.degree. C.
In German Patent Publication No. DE 39 05 080 a slurry is adsorbed to a foamed organic material, such as polyurethane foam, with adsorption thickness of 0.5-3 mm, then it is subjected to a sintering process.
In Japanese Patent Publication No. JP 01 280021 a ceramic composition including aluminium oxide, yttrium oxide, magnesium oxide, silicon oxide and silicon nitride is biscuit fired at 500.degree.-900.degree. C., and sintered at 1400.degree.-1900.degree. C.
Japan Laid Open No:82-10-7215 discloses a composite ceramic including aluminium oxide, cordierite, silicon carbide and silicon nitride, and the ceramic product is used for filtering the carbon powder in the air.
Besides silicon nitride, zirconium oxide can also be used as a material for producing porous ceramic filter that are resistant to temperatures greater than 1400.degree. C. However, the price of zirconium oxide is much higher than that of silicon powder.